Virtually all wells for production of petroleum products which produce such products from a pressurized production zone utilize flow control mechanisms, typically referred to as chokes, for the purpose of controlling the rate of flow from the production zone through the wellhead flow control apparatus. In some cases, chokes are of adjustable nature and incorporate an internal valve element that may be moved relative to a valve seat in order to adjust the rate of flow as desired. In other circumstances, a positive choke structure is provided wherein an internal flow restriction element of fixed nature is positioned such that flowing production fluid must pass through a restricted opening or passage in order to reach a production conduit. Regardless whether adjustable or fixed choke systems are employed, there is typically provided a choke body structure having an access opening in order that access may be gained to the interior of the choke body during manufacture and repair operations. In the case of adjustable choke mechanisms, a valve actuator typically incorporates the closure element and provides for externally controlled movement of the valve element for the purpose of adjusting the rate of flow. In the case of positive choke mechanisms, the closure element is merely provided for closure of the access opening of the choke body structure.
Under low pressure well service conditions, sealing of the closure structure of both adjustable and positive choke mechanisms is not critical. Many different types of seal mechanisms may be successfully employed to achieve optimum sealing at low pressures. In the case of high pressure conditions, the access opening closure structure of many choke mechanisms may require the provision of metal-to-metal sealing as well as providing elastomeric sealing to enhance the sealing capability of the metal-to-metal seal. For example, U.S. Pat. No. 4,047,695 employs a combination metal-to-metal and resilient seals in order to establish closure sealing in an adjustable type choke mechanism. In many cases the choke body is formed to define an internal frusto-conical seat surface that is adapted to be engaged by an external frusto-conical seal surface that is defined on the closure element. A closure retainer structure or bonnet is then threaded downwardly onto the body structure to accomplish forcing of the sealing surface into positive fluid-tight sealing engagement with respect to the seat surface. An example of a choke mechanism of this nature is the type "CH-2" choke mechanism that is manufactured by the McEvoy Company and which is set forth on page 4117 of the composite catalog of Oil Field Equipment & Services of 1976-1977, published by World Oil, Gulf Publishing Company, Houston, Texas.
In many cases, metal sealing rings have been employed in choke mechanisms and other wellhead structures to provide a sealing capability. Examples of metal ring type seals are identified by U.S. Pat. Nos. 3,104,121 of Nordin et al and 3,166,345 of Pinkard. Metal ring-type high pressure seals are also identified in U.S. Pat Nos. 3,927,891 of Larker et al and 4,079,948 of Paureau.
In some cases, metal sealing elements for high pressure sealing are yieldable to some extent as identified by U.S. Pat. Nos. 3,468,566 of Neitzel; 3,490,777 of Emmerson; 3,690,682 of Ferrill; and 4,131,287 of Gunderson et al.